Bent glass sheets are used extensively for vehicle windshields, side windows, and rear windows, as well as in various architectural applications. The bent sheets are also frequently tempered to improve the mechanical strength of the glass. In the United States, tempered bent glass sheets are used on vehicle side and rear windows while annealed bent glass sheets laminated to each other by polyvinyl butyryal are used for vehicle windshields. In other countries, tempered bent glass sheets are used for vehicle windshields as well as for side and rear windows.
Glass sheet press bending is typically performed by pressing a heated glass sheet on a mold, or between complementary curved molds so that the heated glass sheet is bent to conform to the curved shape of the mold or molds. One type of press bending system is disclosed in U.S. Pat. No. 4,661,141, which discloses a glass sheet press bending system including a horizontal conveyor on which glass sheets are conveyed in a generally horizontally extending orientation for heating, and also includes an upper mold having a downwardly facing curved shape located above the conveyor at a bending station. When a glass sheet is conveyed into the bending station, a vacuum drawn at the upper mold and/or upward gas flow from below the conveyor provide a preferred means for supplying a differential gas pressure to the heated glass sheet so as to lift the glass sheet upwardly off the conveyor against the downwardly facing surface of the mold and, preferably, to provide some or all of the bending required to conform the glass sheet to the shape of the mold surface. A lower mold having an upwardly facing, complementary curved shape is typically mounted for movement into position directly below the upper mold, and for subsequent movement upwardly to press bend the heated glass sheet between the upper and lower mold. A transfer mold is thereafter moved horizontally under the upper mold and receives the press bent glass for subsequent transfer therefrom. Normally, the transfer mold is formed as an open-center ring and transfers the press bent glass to a quench station where tempering is performed. This type of press bending system can be utilized with either a gas hearth or a roller-type conveyor.
These systems typically employed a gas jet system generally of the type illustrated in FIG. 8 for providing positive gas pressure on the underside of the glass sheet for lifting the glass from the conveyor and blowing it to the upper forming mold. A control signal opened a "cold valve" solenoid which supplied pressurized air to a heat exchanger (not shown) in the furnace. The pressure level for the pressurized air was set on a pressure regulator. The heated high pressure air was then released to the lift jets by opening a "hot valve" solenoid, thereby lifting the glass from the rolls and blowing it to the forming mold.
This system provided adequate though minimal control of the lift jet forces by providing one or more pre-set forced levels of pressurized air to the system. However, since the action of the lift jets could be controlled only by turning on and off the hot valve or cold valve, the system operated at a single, pre-set pressure. To change the lifting force for bending a different shape, the regulator required adjustment during initial set-up of the system for that shape.
Another problem with the previous system was that upon opening the cold valve, and prior to opening the hot valve, significant excess pressure often built-up in the pressurized gas in the heat exchanger so that, upon opening the hot valve to lift up the glass sheet, the initial surge of pressurized gas was greater than the desired force, causing damage to the protective covering on the mold or breakage of the glass sheet.
Also, if a different lifting force was needed during a different stage in a glass sheet press bending cycle, a second bypass regulator and bypass cold valve would need to be installed. Likewise, additional bypass regulators and bypass cold valves (shown as phantom lines in FIG. 8) would need to be installed for each different lifting force required during the process.